Typically, a fuel cell has a cell stack formed by a number of power generation cells stacked together. With reference to FIGS. 17 to 19, a prior art power generation cell will be described. As shown in FIG. 17, a power generation cell 12 includes a pair of upper and lower frames 13, 14 and an electrode structure 15 between the frames 13, 14. The electrode structure 15 is formed by a solid electrolyte membrane 16, an electrode catalyst layer 17 on the anode side, and an electrode catalyst layer 18 on the cathode side. The anode-side electrode catalyst layer 17 is laid on the upper surface of the electrolyte membrane 16, and the cathode-side electrode catalyst layer 18 is laid on the lower surface of the solid electrolyte membrane 16. A first gas diffusion layer 19 is laid on the upper surface of the electrode layer 17, and a second gas diffusion layer 20 is laid on the lower surface of the electrode layer 18. Further, a first gas passage forming member 21 is laid on the upper surface of the first gas diffusion layer 19, and a second gas passage forming member 22 is laid on the lower surface of the second gas diffusion layer 20. A flat plate-like separator 23 is joined to the upper surface of the first gas passage forming member 21, and a flat plate-like separator 24 is joined to the lower surface of the second gas passage forming member 22.
FIG. 18 is an enlarged perspective view showing a part of the first and second gas passage forming members 21, 22. As shown in FIG. 18, the gas passage forming member 21 (22) is made of a metal lath plate, which has a great number of hexagonal ring portions 21a (22a) arranged alternately. Each ring portion 21a (22a) has a through hole 21b (22b). The ring portions 21a (22a) and the through holes 21b (22b) form gas passages 21c (22c) that meander in a complex manner. Fuel gas (oxidation gas) flows through gas passages 21c (22c) as indicated by arrows.
As shown in FIG. 17, the frames 13, 14 form a supply passage G1 and a discharge passage G2 for fuel gas. The fuel gas supply passage G1 is used for supplying hydrogen gas, which serves as fuel gas, to the gas passages 21c of the first gas passage forming member 21. The fuel gas discharge passage G2 is used for discharging fuel gas that has passed through the gas passages 21c of the first gas passage forming member 21, or fuel off-gas, to the outside.
Also, the frames 13, 14 form a supply passage and a discharge passage for oxidation gas. The oxidation gas supply passage is located at a position corresponding to the back side of the sheet of FIG. 17, and is used for supplying air serving as oxidation gas to the gas passages of the second gas passage forming member 22. The oxidation gas discharge passage is located at a position corresponding to the front side of the sheet of FIG. 17, and is used for discharging oxidation gas that has passed through the gas passages of the second gas passage forming member 22, or oxidation off-gas, to the outside.
As indicated by arrow P in FIG. 17, hydrogen gas is supplied from a hydrogen gas supply source to the first gas passage forming member 21 via the supply passage G1. The air is fed from an air supply source to the second gas passage forming member 22. This causes an electrochemical reaction in each power generation cell to generate power. Since humidifiers (not shown) humidify the hydrogen gas and the oxidation gas, the gases each contain humidifying water (water vapor). The aforementioned electrochemical reaction also produces water in the electrode layer 18 at the cathode side, the gas diffusion layer 20, and the second gas passage forming member 22. The generated water and the humidifying water form water droplets W1 in a portion of the power generation cell 12 at the cathode side. The oxidation off-gas flowing in the gas passage 22a of the gas passage forming member 22 sends the water droplets W1 to the exterior via the discharge passage.
Some of the generated water seeps as seepage water through the solid electrolyte membrane 16 and flows into the electrode layer 17 at the anode side, the gas diffusion layer 19, and the gas passage 21c of the first gas passage forming member 21. The seepage water and the humidifying water form water droplets W in a portion of the power generation cell 12 at the anode side. The oxidation off-gas flowing in the gas passage 21c of the gas passage forming member 21 introduces the water droplets W to the exterior through the discharge passage G2. Patent Document 1 discloses a power generation cell for a fuel cell having the structure shown in FIG. 17.
FIG. 19 is a partial cross-sectional view showing a fuel cell disclosed in Patent Document 2. As illustrated in FIG. 19, the fuel cell has a cathode 49 and a separator 50 that are arranged in a stacked manner. The separator 50 includes a plurality of projections 50a projecting toward the cathode 49. The separator 50 and the cathode 49 form gas passages 52. A deformed member 51 is arranged around each of the projections 50a. Two ends of the deformed member 51 are bent toward the corresponding projection 50a in such a manner as to form obtuse angles R with respect to the cathode 49. As a result, each adjacent pair of walls determining the cross-sectional area of each gas passage 52 form an obtuse angle. This makes it difficult for water droplets to be accumulated in corners of the gas passages 52, thus improving drainage performance of the separator 50.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-87768    Patent Document 2: Japanese Laid-Open Patent Publication No. 2008-21523